1366 A MANUAL OF ANATOMY 



lies close to the median raphe, and on the mesial side of the nucleus 

 of the sixth nerve. Like that nucleus, it lies immediately beneath 

 the grey matter of the pontine part of the floor of the fourth 

 ventricle. 



The fillet (main fillet) lies, as it does in the bulb, ventral to the 

 dorsal longitudinal bundle, iDut in the dorsal part of the pons the 

 two strands are separated by a distinct interval. The fillet occu- 

 pies a broad area in that portion of the lower region of the dorsal 

 part of the pons which is contiguous to the ventral part. The 

 area extends outwards from the median raphe. 



Upper Region of the Dorsal Part of the Pons. — ^This region lies 

 above the level of the trapezium in the ventral part of the pons. 

 The tracts and nuclei of this region, which will be described in this 

 place, are as follows : 



1. Superior peduncle of the cerebellum. 



2. Nuclei of the fifth cranial nerve. 



3. Dorsal longitudinal bundle. 



4. Mesial fillet. 



5. Lateral fiUet. 



The superior peduncle of the cerebellum, after emerging from the 

 corresponding cerebellar hemisphere, lies on the lateral aspect of 

 this region, where it forms the lateral boundary of the upper or 

 pontine part of the floor of the fourth ventricle. Its dorsal aspect 

 is connected with that of its fellow of the opposite side by the 

 superior medullary velum, and ventrally it sinks into the upper 

 region of the dorsal part of the pons. 



The pontine nuclei of the fifth cranial nerve are two motor and two 

 sensory, upper and lower. The motor roots are pontine and Sylvian. 



The pontine motor nucleus is situated close to the superior 

 peduncle of the cerebellum at the lower part of the lateral margin 

 of the upper or pontine part of the fourth ventricle. It lies near 

 the surface, and the axons of its cells form many of the fibres of the 

 motor root of the nerve. The remaining fibres of the motor root 

 represent the descending, mesencephalic, or Sylvian root. These 

 fibres arise from the Sylvian motor nucleus in the grey matter on 

 the lateral aspect of the aqueduct of Sylvius, and as they descend 

 towards the pontine motor nucleus, they lie close to the mesial 

 side of the superior peduncle of the cerebellum. 



The upper sensory nucleus is situated deeply on the outer side of 

 the pontine motor nucleus, and on the ventral aspect of the superior 

 peduncle of the cerebellum. Some of the fibres of the sensory root 

 ascend and terminate in arborizations around the cells of this 

 nucleus. 



The lower sensory nucleus succeeds to the upper sensory nucleus, 

 and is a continuation upwards of the substantia gelatinosa of 

 Rolando. It is elongated and extends into the upper part of the 

 spinal cord to about the level of the second cervical nerve. It lies 

 on the inner side of the spinal or descending sensory root of the 



